The disclosures of Japanese Patent Application Nos. HEI 11-311010 filed on Nov. 11, 1999 and 2000-259842 filed on Aug. 29, 2000 each including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to braking torque control apparatus and method for controlling the braking toque of a brake.
2. Description of the Related Art
Japanese Patent Application No.11-43041 describes an electrically driven brake that controls the rotation of a wheel by driving an electric motor to press a friction engagement member against a brake rotor that rotates together with the wheel and thus achieving friction engagement therebetween. The patent application also describes a braking torque control apparatus for controlling the braking torque applied to the wheel by controlling the current supplied to the motor of the electrically driven brake. The braking torque control apparatus determines a target braking torque based on an amount of braking operation provided by a driving person. Based on the thus-determined target braking torque and a pre-stored relationship between the current supplied to the electric motor and the braking torque, the braking torque control apparatus determines a current to be supplied to the motor. Then, the control apparatus supplies the thus-determined current to the motor. As for the relationship between the supplied current and the braking torque, a plurality of relationships in accordance with coefficients xcexc of friction between the friction engagement member and the brake rotor of the electric brake are pre-stored. A friction coefficient xcexc is estimated based on an actual current supplied to the motor and the actual braking torque. In accordance with the estimated friction coefficient xcexc, one of the relationships is determined. In accordance with the thus-determined relationship, a current to be supplied is determined. Thus, this braking torque control apparatus is able to control the actual braking torque to a magnitude corresponding to the brake operation.
A first object of the invention is to ensure that a braking torque control apparatus will provide braking torque corresponding to brake operation, regardless of the magnitude of the coefficient xcexc of friction between a brake rotor and a friction engagement member. A second object is to allow good performance of brake control in a braking torque control apparatus that feeds back the actual braking torque in order to provide braking torque corresponding to brake operation, even when the braking torque is undetectable at all or cannot be detected with good precision.
In order to achieve aforementioned and other objects, a braking torque control apparatus in accordance with a first aspect of the invention includes a target braking torque determiner that determines a target braking torque based on an amount of brake operation performed by a driving person, an actual braking torque detector that detects an actual braking torque applied to the wheel, and a braking torque-corresponding brake control portion that controls the brake so that the actual braking torque detected by the actual braking torque detector approaches the target braking torque determined by the target braking torque determiner.
In this braking torque control apparatus, the actual braking torque applied to the wheel is detected, and the brake is controlled so that the actual braking torque approaches the target braking torque. Therefore, braking torque can be provided corresponding to brake operation, regardless of the magnitude of the coefficient xcexc of friction between the brake rotor and the friction engagement member.
The brake described herein is, for example, an electric brake that includes an electric actuator such as an electric motor or the like. In the electric brake, the brake is actuated by the electric actuator pressing the friction engagement member against the brake rotor. The pressing force is controlled by controlling the electric energy supplied to the electric actuator. The brake may also be a hydraulic brake that includes (1) a wheel cylinder, and (2) an electromagnetic control valve device capable of electrically controlling the hydraulic pressure on the wheel cylinder. In the hydraulic brake, the wheel cylinder hydraulic pressure is controlled by control of the electromagnetic control valve device. The electromagnetic control valve device and the wheel cylinder form an electric control actuator. The brake may be a disc brake, a drum brake, etc.
The brake may be a drum brake. The drum brake may include a brake drum that rotates together with the wheel, a pair of shoes disposed inside the brake drum, the shoes being retained to an unrotatable backing plate by an anchor so that the shoes are prevented from rotating together with the break drum, and an actuator that brings an outer peripheral surface of each shoe into a friction engagement with an inner peripheral surface of the brake drum by moving the shoes away from each other. In the drum brake, the actual braking torque detector detects the actual braking torque applied to the wheel, based on a force acting on the anchor.
When the two shoes of the drum brake are expanded or moved away from each other, the outer peripheral surfaces of the shoes come into friction engagement with the inner peripheral surface of the brake drum, so that friction occurs therebetween. This friction force is the braking force. Due to this braking force, rotation of the brake drum is restrained. Furthermore, a force corresponding to the braking force acts on the anchor. Therefore, by detecting the force acting on the anchor and multiplying the detected value by a dimensional value specific to the brake construction, a braking torque can be determined.
The braking force increases with increases in the coefficient xcexc of friction between the outer peripheral surfaces of the shoes and the inner peripheral surface of the brake drum if the force expanding the two shoes (which is the actuating force of the drum brake) remains unchanged, that is, if the contact surface pressure between the outer, peripheral surfaces of the shoes and the inner peripheral surface of the brake drum remains unchanged. In the drum brake in particular, the braking force is more likely to be affected by the friction coefficient than in the disc brake, due to the servo effect that the friction force acting on the brake shoes of the drum brake increases the contact surface pressure between the brake shoes and the brake drum. The braking force, that is, the braking torque, varies to a considerably great extent even if it is provided by a fixed amount of brake operation. Therefore, if the braking torque-corresponding brake control is performed on the drum brake, the actual braking torque can be provided in good correspondence to the brake operation (target braking torque), regardless of the magnitude of the friction coefficient.
The brake may also be a duo-servo type drum brake. The duo-servo type drum brake includes a brake drum that rotates together with the wheel, a pair of shoes disposed inside the brake drum, the shoes being retained to a backing plate in such a manner that first end portions of the shoes that face each other are engaged with an anchor and the shoes are prevented from rotating together with the brake drum, a strut provided so that the strut interconnects second end portions of the shoes so as to transmit a force acting on one of the shoes to another one of the shoes, and an actuator that brings an outer peripheral surface of each shoe into a friction engagement with an inner peripheral surface of the brake drum by moving the first end portions of the shoes away from each other. In the duo-servo type drum brake, the actual braking torque detector detects the actual braking torque applied to the wheel, based on a force acting on at least one of the anchor and the strut.
If the first end portions of the shoes are expanded or moved away from each other, the outer peripheral surfaces of the shoes come into friction engagement with the inner peripheral surface of the brake drum, so that friction force occurs therebetween. The drag force based on the friction produced on one of the shoes and the expansion force caused by the actuator are transmitted from the second end portion of that shoe to the second end portion of the other shoe via the strut. Therefore, the second shoe is pressed against the drum by the sum of the drag force and the expansion force, so that a greater friction occurs on the second shoe than on the first shoe. The force applied from a first one of the shoes to the strut becomes an output that is increased, by the servo effect of the first shoe, from the expansion force applied to the first end portion of the first shoe. The force applied from the second shoe to the anchor becomes an output further increased from the output of the first shoe by the servo effect of the second shoe. Therefore, based on the force acting on the anchor, the braking torque of the entire duo-servo type drum brake can be detected. However, the braking torque determined based on the force acting on the strut merely includes the braking torque produced by one of the shoes, and does not include the braking torque produced by the other shoe. Therefore, the braking torque produced by the other shoe needs to be taken into consideration in order to determine the braking torque of the entire duo-servo type brake drum.
Thus, among the drum brakes, a duo-servo type drum brake receives a particularly great effect of the friction coefficient xcexc on the braking torque since the output from one of the two shoes of the duo-servo type drum brake becomes an input to the other shoe so that a double servo effect is achieved. Therefore, it is particularly effective to apply the invention to the duo-servo type drum brake so that the braking torque corresponding to brake operation can be provided regardless of the magnitude of the friction coefficient xcexc.
The actuator may be an electric actuator that is actuated upon supply of an electric energy. The electric actuator includes, for example, an electric motor. Through control of the electric energy supplied to the electric motor, it is possible to directly control the force for expanding the two shoes, that is, the brake actuating force.
The braking torque control apparatus in accordance with the first aspect of the invention may further include a target actuating force determiner that determines a target actuating force of the brake based on the amount of brake operation, an actual actuating force detector that detects an actual operating force of the brake, an actuating force-corresponding brake control portion that controls the brake so that the actual actuating force detected by the actual actuating force detector approaches the target actuating force determined by the target actuating force determiner, and a selecting portion that selects one of the actuating force-corresponding brake control portion and the braking torque-corresponding brake control portion based on a state of a vehicle.
The braking torque control apparatus in accordance with the first aspect of the invention includes an electric actuator that generates an actuating force in accordance with a current supplied, and an electric brake that is driven by the electric actuator to apply the braking torque to the wheel. The apparatus controls the braking torque applied to the wheel by controlling the electric actuator. The braking torque control apparatus may further include an amount-of-brake-operation detector that detects an amount of brake operation performed by a driving person, an actual braking torque detector that detects an actual braking torque applied to the wheel, a braking torque-corresponding brake control portion that determines a target braking torque based on the amount of brake operation detected, and that feeds back the actual braking torque detected, and that controls a current flowing through the electric actuator so that the actual braking torque detected becomes equal to the target braking torque determined, an actuating force-corresponding brake control portion that determines a target value of current based on the amount of brake operation detected, and that performs a control such that a current equal to the target value of current determined is caused to flow to the electric actuator, and a selecting portion that selects one of the actuating force-corresponding brake control portion and the braking torque-corresponding brake control portion, based on a state of a vehicle in which the braking torque control apparatus is installed.
According to this construction, if the braking torque-corresponding brake control portion is selected, the brake is controlled by the braking torque-corresponding brake control portion. If the actuating force-corresponding brake control portion is selected, the brake is controlled by the actuating force-corresponding brake control portion. The selection of one of the braking torque-corresponding brake control portion and the actuating force-corresponding brake control portion is made based on a state of the vehicle.
The selecting portion switches from the control by the braking torque-corresponding brake control portion to the control by the actuating force-corresponding brake control portion when a state where the actual braking torque is substantially undetectable by the actual braking torque detector is established.
If the control of bringing the actual braking torque toward the target braking torque is performed when the actual braking torque is substantially undetectable, that is, when the actual braking torque cannot be detected with good precision, there is a danger of the brake actuating force becoming excessively great. In contrast, if the actual brake actuating force is controlled so as to approach the target actuating force when the actual braking torque is substantially undetectable, the danger of the actuating force becoming excessively great can be avoided.
The state where the actual braking ,torque is substantially undetectable by the braking torque detector may be detected based on a quantity of decelerating state that indicates a decelerating state of the vehicle, for example, the vehicle body speed, the wheel speed, the braking torque, a differential of any one of the vehicle body speed, the wheel speed, and the braking torque, etc.
A time when the state where the actual braking torque is substantially undetectable by the actual braking torque detector is established may be a time when one of a wheel speed and a vehicle body speed becomes equal to or lower than a set speed.
When the vehicle body speed or the wheel speed is at most the set speed, the actual braking torque becomes so small that the actual actuating force cannot be detected with good precision. The aforementioned set speed may be set to such a magnitude that the vehicle can be considered stopped. In this case, the vehicle body speed may be calculated from the speeds of the wheels, the rotational speed of the output shaft of the transmission, etc. In a vehicle equipped with an ABS (anti-lock brake system), the vehicle body speed may be a vehicle speed estimated by the ABS. The wheel speed is calculated from the rotational speed of the wheel. When the wheel speed (or vehicle speed) is near 0 km/h, the intervals between pulses generated in accordance with the rotation of the wheel become large, so that the precision in detecting the wheel speed becomes low. Therefore, it may be advisable that when the wheel speed is equal to or less than a predetermined low speed (e.g., 5 km/h), a substantially stopped state be considered established, and the control be switched from the control by the braking torque-corresponding brake control portion to the control by the actuating force-corresponding brake control portion. It is to be noted that if the detection precision of the wheel speed (or vehicle speed) is high when the wheel speed is near 0 km/h, the predetermined low speed can be set to a value that is very close to xe2x80x9c0xe2x80x9d.
Furthermore, the time when the state where the actual braking torque is substantially undetectable by the actual braking torque detector is established may be a time when the wheel becomes a locked state.
The actual braking torque cannot be detected with good precision when the wheel is in the locked, as is the case when the wheel speed is at most the set speed. The locked state substantially the same as the slipping state of the wheel. Therefore, the aforementioned locked state may be replaced with the slipping state.
Furthermore, the time when the state where the actual braking torque is substantially undetectable by the actual braking torque detector is established may be a time when a deviation between the target braking torque determined and the actual braking torque detected by the actual braking torque detector becomes equal to or greater than a predetermined value.
Immediately before the wheel sped becomes xe2x80x9c0xe2x80x9d, the actual braking torque detected by the actual braking torque detector sharply decreases whereas the determined target braking torque remains large. Therefore, the deviation between the actual and target braking torques can be used as a basis for detecting that the wheel speed has become substantially xe2x80x9c0xe2x80x9d. In this case, the actual braking torque cannot be detected with good precision, as in the case where the wheel speed is at most the set speed. Furthermore, if besides the condition that the deviation between the actual and target braking torques is at least a predetermined value, a new condition that the aforementioned condition continues for at least a predetermined time is added, the state where the actual braking torque is undetectable can be more reliably detected.
Furthermore, in addition to or instead of the condition that the deviation between the actual and target braking torques is at least the predetermined value, a condition that the decreasing rate of the actual braking torque detected by the actual braking torque detector is at most a predetermined negative value may be used to detect the state where the actual braking torque is undetectable.
Still further, the time when the state where the actual braking torque is substantially undetectable by the actual braking torque detector is established may be a time when the actual braking torque detected by the actual braking torque detector starts to oscillate.
When the vehicle is stopped due to braking torque, the actual braking torque detected by the actual braking torque detector starts to oscillate due to backlash. Therefore, based on oscillation of the detected actual braking torque, too, it is possible to detect that the wheel speed has become substantially equal to xe2x80x9c0xe2x80x9d. In this case, too, the actual braking torque is undetectable with good precision, as in the case where the wheel speed is equal to or less than the set speed. As for the detection of oscillation of the actual braking torque, detection of oscillation of the value detected by a braking torque sensor about a reference value (e.g., xe2x80x9c0xe2x80x9d) may be sufficient. The reliability of the detection can be enhanced by counting the number of times of the detected value passing the reference value and detecting the state where the actual braking torque is undetectable when the counted number of times reaches or exceeds a predetermined number.
Still further, the time when the state where the actual braking torque is substantially undetectable by the actual braking torque detector is established may be a time when the actual braking torque detected by the actual braking torque detector becomes equal to or less than a predetermined value.
Immediately before the wheel speed becomes xe2x80x9c0xe2x80x9d, the actual braking torque detected by the actual braking torque detector sharply decreases as mentioned above. Therefore, based on the detected actual braking torque being equal to or less than the preset value, it is also possible to detect that the wheel speed has become substantially equal to xe2x80x9c0xe2x80x9d. In this case, too, detection of the actual braking torque with good precision is impossible, as in the case where the wheel speed is at most the set speed. If besides the condition that the actual braking torque detected by the actual braking torque detector is at most the predetermined value, a condition that the deviation between the detected actual braking torque and the determined target braking torque is at least a predetermined value is added, the precision in detecting the state where the actual braking torque is undetectable by the actual braking torque detector increases.
The time when the state where the actual braking torque is substantially undetectable by the actual braking torque detector is established may be a time when a fore-to-aft acceleration detected by a fore-to-aft acceleration sensor installed in the vehicle becomes equal to or less than a predetermined value. As the predetermined value, a very small value close to xe2x80x9c0xe2x80x9d is adopted. In this manner, too, it is possible to detect that the wheel speed has become substantially equal to xe2x80x9c0xe2x80x9d. In this case, too, the actual braking torque is undetectable with good precision, as in the case where the wheel speed is at most the set value.
The fore-to-aft acceleration detected by the fore-to-aft acceleration sensor oscillates due to backlash at the time of a stop of the vehicle. Therefore, as in the case of the braking torque, it may be advisable to detect the state where the actual braking torque is undetectable, based on oscillation of the fore-to-aft acceleration. The number of oscillations of the detected fore-to-aft acceleration may also be added as a condition for the detection.
Furthermore, the aforementioned conditions may be suitably combined to detect the state where the actual braking torque is undetectable, that is, a state where the wheel has become substantially equal to xe2x80x9c0xe2x80x9d.
The selecting portion may select the braking torque-corresponding brake control portion when the actuating force of the brake is not excessively great relative to the coefficient of friction between the tire and a road surface.
Furthermore, the selecting portion may select the braking torque-corresponding brake control portion when the slip state of the wheel is better than a preset state.
The friction occurring between the tire and a road surface increases with increases in the brake actuating force if the friction is within a range whose maximum is a friction that corresponds to the maximum friction coefficient between the tire and the road surface. That is, this range is a range where the braking torque increases with increases in the brake actuating force. If the braking torque-corresponding brake control is performed when the friction is within this range, the actual braking torque can be brought close to the target braking torque.
After the friction on the road surface reaches the friction corresponding to the maximum friction coefficient of the road surface, the friction does not increase any further. Therefore, if in that case, the brake actuating force is increased, the friction does not increase, but decrease. If in this state, the braking torque-corresponding brake control is performed so that the brake actuating force is increased due to the actual braking torque being insufficient for the target braking torque, the wheel locking tendency becomes stronger and the braking torque becomes smaller.
During a state where the friction on the road surface is greater than the magnitude corresponding to the maximum friction coefficient, it is desirable that the anti-lock control be performed. In the anti-lock control, the brake actuating force is controlled so that the slip state of the wheel is kept in an appropriate state. As a result, a substantially maximum braking torque can be obtained. However, if an anti-lock control apparatus is not provided, or if although an anti-lock control apparatus is provided, the anti-lock control cannot be performed for any reason, it is desirable that the actuating force-corresponding brake control be performed. If the actuating force-corresponding brake control is performed, inappropriate performance of the braking torque-corresponding brake control as mentioned above is avoided.
The selecting portion may select the actuating force-corresponding brake control portion if the slip state of the wheel is worse than a set state and the vehicle running speed is less than an anti-lock control-prohibiting speed.
If the slip state is worse than the set state, it is desirable that the anti-lock control be performed. However, if the vehicle running speed is less than the anti-lock control-prohibiting speed, the anti-lock control is not performed. In this case, it is desirable that the actuating force-corresponding brake control, not the braking torque-corresponding brake control, be performed.
It is also possible to perform the actuating force-corresponding brake control when the anti-lock control ends due to the slip state being better than the set state and the vehicle running speed being less than the anti-lock control-prohibiting speed. According to the braking torque control apparatus that detects that the state where the actual braking torque is undetectable by the actual braking torque detector is established, on condition that the wheel speed is at most a predetermined set speed, the braking torque-corresponding brake control is supposed to be performed if the wheel rotational speed is at least the set speed when the anti-lock control is ended due to a reduction in the slip of the wheel and the vehicle running speed decreasing below the anti-lock control-prohibiting speed. However, when the running speed is less than the anti-lock control-prohibiting speed, it is highly likely that the vehicle will be stopped within a short time. As the wheel rotational speed decreases to or below the set speed, the actuating force-corresponding brake control is performed. Thus, the control is switched in the order of the anti-lock control, the braking torque-corresponding brake control, and the actuating force-corresponding brake control, within a short time, which is not desirable. Therefore, if a design is made such that the actuating force-corresponding brake control is performed when the anti-lock control is ended as the aforementioned conditions are met, the switching of the mode of the brake control within a short time can be avoided.
At a time of switching from a control performed by the braking torque-corresponding brake control portion to a control performed by the actuating force-corresponding brake control portion, the selecting portion may gradually perform the switching.
Therefore, even if at the time of the control by the braking torque-corresponding brake control portion to the control by the actuating force-corresponding brake control portion, there is a difference between the braking torques caused by the controls of the two control portions, the braking torque actually applied to the wheel is gradually changed, so that a driving person will receive no uncomfortable feeling.
In this case, the control gain of the braking torque achieved by the braking torque-corresponding brake control portion may be gradually decreased and the control gain of the braking torque achieved by the actuating force-corresponding brake control portion may be gradually increased, as time elapses after the actual braking torque becomes substantially undetectable. Gradual decreases in the control gain of the braking torque by the braking torque-corresponding brake control portion and gradual increases in the control gain of the braking torque by the actuating force-corresponding brake control portion may also be caused with increases in the deviation between the determined target braking torque and the actual braking torque detected by the actual braking torque detector, instead of the elapse of time.
The brake may include a disc brake that is provided for a front wheel and that brings a friction engagement member into friction engagement with a disc rotor that rotates together with the front wheel, and a drum brake that is provided for a rear wheel and that brings a friction engagement member into friction engagement with an inner peripheral surface of a brake drum that rotates together with the rear wheel. In this case, the braking torque control apparatus prohibits control of the drum brake of the rear wheel by the braking torque-corresponding brake control portion if the anti-lock control is performed on at least one of the front and rear wheels.
As mentioned above, performance of the control by the braking torque-corresponding brake control portion is not desirable when the anti-lock control is being executed.
Furthermore, the braking torque control apparatus may prohibit actuation of the drum brake of the rear wheel when the vehicle running speed is at most the anti-lock control-prohibiting speed.
When the vehicle running speed is less than the anti-lock control-prohibiting speed, great braking torque is unnecessary in most cases. During a state where the anti-lock control is performed (a state where the wheel rotational speed is low), the braking torque-corresponding brake control cannot be performed with good precision. Furthermore, the rear wheel brake produces smaller braking torque than the front wheel brake.
Thus, if the braking torque cannot be controlled with good precision when large braking torque is not needed, it is desirable in some cases that actuation of the rear brake be prohibited.
Aspects of the invention are not limited to the braking torque control apparatus described above. Other aspects of the invention are, for example, a vehicle equipped with a braking torque control apparatus, and a control method for a braking torque control apparatus.